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Use a VNA/analyzer scalar reading or a matched pair of forward and reflected power readings.
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Forward power must be positive. dBm and dBW values are converted to watts before the mismatch calculation.
Review bands are practical context aids, not equipment limits or pass/fail standards.
Use extra digits for VNA marker comparisons; measurement uncertainty still comes from the instrument and setup.
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Bench interpretation

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What the number does not prove

This scalar conversion does not include reflection phase, feedline loss, calibration uncertainty, antenna efficiency, device power limits, bandwidth, or fault location. Use the applicable instrument procedure and equipment limits before transmitting or signing off a measurement.

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A neat 1.5:1 VSWR reading can be reassuring, but it is only one view of an RF mismatch. The same reflection can be written as about 14 dB return loss, a reflection coefficient magnitude of 0.2, or 4% reflected power. Converting among those forms makes measurements from antennas, vector network analyzers, data sheets, and directional wattmeters comparable.

Mismatch occurs when a source, transmission line, connector, fixture, matching network, or load does not present the expected impedance. Part of the incident wave returns toward the source. In a fixed-impedance scalar model, voltage standing wave ratio (VSWR), return loss, reflection coefficient magnitude, reflected power, and mismatch loss all describe that same returned share.

VSWR
A ratio from 1:1 upward. Values closer to 1 indicate less reflection.
Return loss
A positive decibel value. Higher values indicate less reflection.
Reflection coefficient
The magnitude of reflected voltage wave divided by incident voltage wave, written as |Γ|.
Mismatch loss
The power loss caused by reflection alone, excluding cable, connector, filter, and antenna losses.

Scalar mismatch cannot locate a fault or reveal its phase. A lossy feedline can make transmitter-end VSWR look better because the reflected wave is attenuated on the return trip, even while useful power becomes heat. A single-frequency value also says nothing about performance across a wider band.

Useful RF records include frequency, reference plane, calibration state, forward power, coupler direction, and equipment limits alongside the converted number. Without that context, a correct conversion can still support the wrong tuning or safety decision.

How to Use This Tool:

Enter the measurement form already supplied by the instrument or data sheet; all other mismatch quantities derive from it.

  1. Set Start from to VSWR, return loss, reflection coefficient magnitude, reflected power percent, or forward/reflected power meter.
  2. Enter the primary value and a positive Forward power. In power-meter mode, enter reflected power in the same selected unit and keep it no greater than forward power.
  3. Choose W, mW, dBm, or dBW, then select the Review context that matches antenna work, a general bench, high-power transmission, or a precision VNA note.
  4. Use Display precision to change visible digits only. It does not change the full-precision calculation.
  5. Read the Conversion ledger and Bench note. Correct any range error before relying on the power split or context label.

Interpreting Results:

Better scalar matches move in opposite directions on the common scales: return loss increases while VSWR, |Γ|, reflected power, and mismatch loss decrease. At 20 dB return loss, |Γ| is 0.1 and reflected power is 1%. At 1.5:1 VSWR, |Γ| is 0.2 and reflected power is 4%.

  • Reflected power and Delivered power split the entered forward power under the scalar reflection model; they sum to the forward value.
  • Mismatch loss is reflection-only loss. It does not include feedline attenuation, insertion loss, antenna efficiency, heating, or transmitter foldback.
  • The review label uses the selected context's return-loss thresholds. It is a planning cue, not a universal pass mark or equipment rating.
  • Before applying high power or accepting precision data, verify frequency, calibration, reference plane, connector condition, and the manufacturer's reflected-power limits.

Technical Details:

The magnitude of the voltage reflection coefficient, |Γ|, is the shared state behind every conversion. A perfect match has |Γ| = 0, VSWR 1:1, no reflected power, zero mismatch loss, and infinite return loss. Complete reflection has |Γ| = 1, infinite VSWR, 100% reflected power, infinite mismatch loss, and 0 dB return loss.

Formula Core:

The selected starting measurement first becomes |Γ|. The remaining values then follow from the same coefficient.

|Γ| = S−1S+1 |Γ| = 10−RL/20 |Γ| = R100 |Γ| = PrPf S = 1+|Γ|1−|Γ| RL = −20log10(|Γ|) R = 100|Γ|2 ML = −10log10(1−|Γ|2)

Here S is VSWR, RL is return loss in dB, R is reflected power percent, ML is mismatch loss in dB, and Pr and Pf are reflected and forward power in the same unit. Reflected watts equal Pf|Γ|²; delivered watts equal Pf(1 − |Γ|²).

Power is normalized to watts before the split. Milliwatts divide by 1,000. dBm converts as 10(dBm − 30)/10 W, while dBW converts as 10dBW/10 W. Full precision is retained through calculation; the selected display precision affects formatting only.

Starting basis Inclusive valid range
VSWR1 through 1,000,000
Return loss0 through 300 dB
|Γ|0 through 1
Reflected power0 through 100%
Power meterReflected power from 0 through forward power, using the same unit
Forward powerGreater than 0 and no more than 1,000,000,000,000 W after conversion

Rule Core:

Each label uses the first return-loss band whose lower bound is met. Lower bounds are inclusive.

Review context Return loss lower bound Label
Antenna field check20 dBExcellent match
Antenna field check13.9 dBGood match
Antenna field check9.54 dBUsable match
Antenna field check6.02 dBMarginal match
Antenna field check0 dBPoor match
General RF bench26 dBPrecision match
General RF bench20 dBGood bench match
General RF bench13.9 dBReviewable match
General RF bench9.54 dBHigh reflection
General RF bench0 dBMismatch fault
High-power transmitter review26 dBVery low reflection
High-power transmitter review20 dBLow reflection
High-power transmitter review16 dBWatch reflected watts
High-power transmitter review9.54 dBProtection review
High-power transmitter review0 dBDo not rely on this match
Precision VNA note30 dBExcellent VNA match
Precision VNA note26 dBGood VNA match
Precision VNA note20 dBUsable VNA match
Precision VNA note13.9 dBRe-check fixture
Precision VNA note0 dBWeak precision match

The thresholds change only the review label and note. They do not alter |Γ|, VSWR, return loss, reflected percentage, mismatch loss, or the watt split.

Accuracy and Safety Notes:

The conversion is a scalar, steady-state mismatch model. Measurement uncertainty, coupler directivity, calibration drift, connector repeatability, cable attenuation, modulation peaks, duty cycle, and load changes are outside the equations.

  • Use instrument and transmitter protection limits instead of a generic profile label for high-power decisions.
  • Keep the reference plane consistent when comparing runs; moving cables or adapters changes what the measurement includes.
  • All calculations run in the browser. No RF reading or power value is sent to a server-side calculator.

Worked Examples:

Antenna check at 1.5:1 and 50 W

A 1.5:1 VSWR gives |Γ| = 0.2, return loss about 13.9794 dB, 4% reflected power, and about 0.1773 dB mismatch loss. At 50 W forward power, 2 W is reflected and 48 W is delivered by the scalar model. The antenna profile labels it Good match because 13.9794 dB meets the inclusive 13.9 dB boundary.

Bench reading at 20 dB and 30 dBm

Return loss of 20 dB gives |Γ| = 0.1, VSWR about 1.2222:1, 1% reflected power, and about 0.04365 dB mismatch loss. Forward power of 30 dBm is 1 W, so the split is 0.01 W reflected and 0.99 W delivered. The general RF profile labels the result Good bench match.

References: